Fluid control of deeply subducted carbonate rocks and diamond formations by Intraslab UHP metasomatism - Modeling by the Kokchetav Massif

Abstract:

Deep continental subductions are an input for material cycling from surface to deep mantle. The Kokchetav UHPM rocks are the best samples and evidence to understand chemical processes in subducting materials. Transportation of H₂O and CO₂, is the most important role of the deep continental subduction. Silicate rocks are H₂O reservoirs as hydrate minerals and carbonate rocks are CO₂ reservoirs during subduction. The timings of dehydrations in silicate rocks and decarbonations in carbonate rocks are different. Dehydrations precede decarbonations and H₂O play as a trigger for decarbonations, which are difficult to occur under dry conditions in P-T range of UHP metamorphism. The amount of H₂O infiltrating in carbonate rocks controls the amount of CO₂ carried into the mantle.

H₂O-bearing fluid plays an important role for diamond formation during subduction of continental materials. Diamonds form and dissolve in subducting materials through H₂O fluid. In UHP dolomite marble, diamonds formed at two different stages and 2nd stage growth was from H₂O fluid. The diamond at 2^nd stage growth has light carbon isotope compositions, -17 to -27 ‰, whereas 1^st stage diamond has -8 to -15 ‰. The light carbon of 2^nd stage could be organic carbon in gneisses carried by fluid; dissolution of diamond in gneisses had occurred. H₂O fluid infiltration into dolomite marble caused the change of carbon solubility in fluid to precipitate abundant fine-grained (10-20 mm) diamonds quickly.

During deep continental subductions, the abundant carbonate remains and are carried to the mantle. In the case of calc-silicate rocks, for example Grt-Cpx rock of the Kokchetav, the carbonate mode is small; therefore, even a small amount of H₂O can decompose all amount of carbonate to form Grt and Cpx which contain several hundreds to 1,000 ppm order of water (OH and H₂O), as new water reservoirs. UHP metasomatism with skarn mineral formation causes the swapping of H₂O carrier from hydrate minerals in silicate rocks to NAMs in calc-silicate rocks to expand the life of H₂O transportation into mantle much longer.

CO₂ transportation is controlled by H₂O in subducted materials, and poor amount of H₂O expands the volume of CO₂ transported into the deep mantle as carbonate. Summarizing these, “Intraslab UHP metasomatism” was proposed.